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Global Health Matters

September - October, 2007  |  Volume 6, Issue 5

Underestimated Parasite Prevalence in Malaria Test Populations Studied by Fogarty Research Team

The anopheles mosquito is a
vector that helps to spread
malaria. (Photo: CDC)

The intensity of malaria transmission is often measured by looking at the fraction of individuals infected at a given point in time. However, malaria infections in individuals are dynamic, leading to uncertainty about whether a cross-sectional survey that represents a single snapshot in time is a useful representation of a temporarily complex process.

In a recently published analysis in the Journal of Tropical Medicine and Hygiene, a research team funded and led by a Fogarty International Center scientist examines the impact of parasite density fluctuations on the measurement of parasite prevalence. The results show that parasite prevalence may be underestimated by 20 percent or more, depending on the sensitivity of parasite detection.

The endemicity of malaria is often characterized by the prevalence of infection in a cross-section of the exposed population. However, point-prevalence surveys cannot capture or account for the dramatic fluctuations in parasite density in infected individuals, which can periodically fall below detectable levels.

The researchers examined the consequences of taking a static snapshot of dynamic processes, by constructing test populations of infected individuals from daily charts of observations in individuals undergoing malaria therapy for neurosyphilis, and measuring prevalence on random days. The prevalence in the test group was known to be one but fluctuations in detectable parasitemia led to a more than 20 percent underestimation of parasite prevalence. When the test population was altered to include uninfected individuals, the underestimation of true parasite prevalence scaled linearly with true parasite prevalence. As expected, declining sensitivity of detection leads to greater underestimation of parasite prevalence.

The natural history of infection in an individual depends on previous exposure. Therefore, the types and distribution of dynamic profiles of infection in a population will be related to the local transmission intensity. The research results depend very strongly on the amount of time that parasite densities remain at, or near, the limit of detection during an infection. Individuals with little or no prior exposure to malaria, either very young children or people living in low-transmission areas, are more likely to experience high parasitemias with large fluctuations, similar to the individuals in the study's test population who were experiencing their first malaria infection.

Thus, this test population is an appropriate comparison for studies in which parasite prevalence in children younger than ten years of age is used to characterize transmission. Persistent, low-density infections are more likely to occur in semi-immune adults and older children. Results suggest that parasite prevalence measured in semi-immunes may be underestimated to an even greater extent if they do indeed maintain very low-density infections.

This study was funded by Fogarty and the Division of Parasitic Diseases, Center for Disease Control and Prevention.

Parasite Prevalence: A Static Measure of Dynamic Infections, Prudhomme-O'Meara W, Collins WE, McKenzie FE. Am. J. Trop. Med. Hyg., 77 (2), 2007, pp. 246-249.

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